This invention relates to a method and apparatus for recording zeroth order light from a laser source, acousto-optically modulated in accordance with an information signal, on a heat sensitive recording medium having a threshold deformation level.
In the prior art, acousto-optical or electro-optical type modulators are empolyed to intensity-modulate laser light with a signal frequency higher than several KHz, with the acousto-optical type modulator being generally superior in respect of light extinction ratio and efficiency as compared to the electro-optical type modulator. An acousto-optical type modulator utilizes an optical elasticity effect whereby an acoustic wave varies the refractive index of a medium. More specifically, in an acousto-optical type modulator layers having different refractive indices are periodically formed in the medium, and light is diffracted through the layers to effect modulation.
Light modulation by an acousto-optical type modulator will be described with reference to FIGS. 1(a) and 1(b), wherein the abscissas represent time and the ordinates represent the quantity of light, the scale graduations of the two figures being equal.
In FIG. 1 three pulses of modulated light are shown. The variation of diffracted or "first order" is shown in FIG. 1(a), while the variation of undiffracted light, or "zeroth order" light, is shown in FIG. 1(b). The phase of the first order light is, of course, opposite to that of the zeroth order light. In FIG. 1(b) the undiffracted light d is superposed on light l, which is not affected by the modulation, to form the composite zeroth order light signal. Zeroth order light has a very unsatisfactory light extinction ratio due to the presence of the unmodulated light l. If the zeroth order light is empolyed as the recording signal a fogging phenomenon will take place, and accordingly the resultant record image is poor in contrast. The first order light, on the other hand, has a very high light extinction ratio, on the order of 1000:1, and accordingly excellent iage contrast can be achieved.
Heretofore, in conventional laser recording apparatuses using an acousto-optical type of modulator, only the first order light has been employed for recording. That is, no laser recording apparatus of this type has yet employed the zeroth order light as the recording signal.